Generally, when completing a subterranean well for the production of fluids, minerals, or gases from underground reservoirs, several types of tubulars are placed downhole as part of the drilling, exploration, and completions process. These tubulars can include casing, tubing, pipes, liners, and devices conveyed downhole by tubulars of various types. Combinations of different tubulars may be lowered into a well for a multitude of purposes.
When placing any type of tubular downhole there is a risk that it can get stuck in the well. This can happen for several reasons including: the well has partially collapsed, operator error, or due to the geometry of the drilling path. Once the tubular becomes stuck, a variety of non-destructive means are available for the operator of the rig to try and free the tubular. These include rotating the tubular, jolting the tubular, or simply pulling up on the tubular until it comes free. However, if these options are unsuccessful then the operator might have to resort to using a cutting or severing tool such as a jet cutter to cut the tubular.
Tubulars may also be cut in abandonment operations. Abandonment operations are increasingly subject to regulations for minimizing the long term environmental impact of abandoned wells. An operator will often times have to remove miles of tubulars while contending with cemented equipment, damage in the wellbore, or other unforeseen difficulties. The jet cutter is a critical tool that allows the operator to cut and retrieve tubulars from the well. The demand for cleaner abandoned wells, in conjunction with the growing number of idle wells in general, is a driving force in the market for jet cutters.
A jet cutter is an explosive shaped charge that has a circumferential V-type shape. The explosive is combined with a liner. The components are all contained in a housing. The jet cutter is lowered to the point where the separation of the tubular is desired. When the jet cutter is detonated, it will generate a jet of high energy plasma, typically in a 360 degree arc, that will severe the tubular. Afterwards, the upper portion of the tubular is pulled out of the well. Then the operator can use a fishing tool to remove the lower portion of the tubular.
While other types of tubular cutters are available, including mechanical cutting devices and chemical cutters, one application of this invention is on explosive shaped charge jet cutters that are widely used throughout the oil industry.
A shaped charge is a term of art for a device that when detonated generates a focused explosive output. This is achieved in part by the geometry of the explosive in conjunction with a liner in the explosive material. Many materials are used for the liner, some of the more common metals include brass, copper, tungsten, and lead. When the explosive detonates the liner metal is compressed into a super heated, super pressurized jet that can penetrate metal, concrete, and rock.
The shaped charge explosives in jet cutters are typically detonated by a booster explosive located in a central cavity coaxial with the shaped charge. This booster is typically detonated from the top, causing a detonation wave to travel down the booster longitudinally. The longitudinal component of the detonation can cause deflection of the shaped charge jet from the ideal, purely radial, direction. The longitudinal deflection of the cutting jet can reduce the effectiveness of the cutter and cause a curved or cupped cut in the target tubular. A device that could detonate a jet cutter booster along its entire length simultaneously would remove any off-axis components of the shaped charge jet.